Method for preparing light-sensitive silver halide emulsions suitable for print-out recording materials



United States Patent Office 3,260,605 Patented July 12, 1966 3 260,605METHOD FOR PREPiAlRlNG LIGHT-SENSITIVE SILVER HALIDE EMULSIUNS SUITABLEFOR PRINT-OUT RECORDING MATERIALS Edward Arthur Sutherns, Harrow,Middlesex, England, assignor to Eastman Kodak Company, Rochester, N.Y.,a corporation of New Jersey No Drawing. Filed Sept. 5, 1962, Ser. No.221,428 8 Claims. (Cl. 96108) This invention relates to silver halidephotography, and, more particularly, to a method of preparingphotographic silver halide emulsions, especially those useful inprint-out photography.

Many methods of preparing photographic silver halide emulsions have beenpreviously described and among the most ancient of these methods arethose concerned with the preparation of silver halide emulsionsexhibiting print-out characteristics. Print-out emulsions aredistinguished from developing-out emulsions in that the former produce avisible image merely upon exposure to actinic radiation, whereasdeveloping-out emulsions produce a visible image upon chemicaldevelopment with a reducing agent, such as hydroquinone.

Print-out materials for recording the path of an oscillograph spot havealso been described in the prior art. Print-out papers, known asdirect-writing materials, are particularly useful for this purpose sincethey provide ready access to the recorded information withoutconventional processing. In general, such oscillograph recordingexposures do not ordinarily give a visible image. The record trace ismade visible by a photodevelopment technique wherein a subsequentexposure to low intensity lighttusually diffuse) is given.

However, many of these known print-out Papers have one or more of thefollowing defects:

(1) The background where no exposure occurred in the oscillograph mayprint out to a substantial density, and since the density in the exposedareas may be quite low, there is inadequate discrimination between thetrace density and the background density. This may be no more than 0.1to 0.2 density unit.

(2) Satisfactory discrimination may be only obtained by restricting theintensity of the difiuse low intensity exposure so that the time neededto give an adequate trace density, termed access time, may be a minuteor more. Use of higher intensity for the purpose of shortening theaccess time produces reduced discrimination.

(3) Prolonged exposure, such as may be caused by leaving the paper outin a normal workroom reduces the discrimination either by causing a risein the density of the background or by reducing the trace density, whichmay fade completely.

(4) The range of frequencies which can be recorded, even byoscillographs with very powerful light sources, is restricted byemulsion sensitivity, which determines writing speed.

According to my invention, there is provided a method of making alight-sensitive photographic silver halide emulsion exhibitingparticularly useful print-out characteristics and. which avoids most orall of the aforementioned defects.

It is, therefore, an object of my invention to provide an improvedmethod of preparing photographic emulsions. Another object is to providephotographic emulsions exhibiting improved print-out characteristics.Still another object is to provide print-out silver halide emulsionsexhibiting especially useful stability following thepost-latensification exposure. Other objects will become apparent from aconsideration of the following description and examples.

The method of my invention comprises precipitating silver halide in anaqueous medium containing a protein peptizer, such as gelatin or agelatin derivative. The precipitation of the silver halide is mostadvantageously effected by the double-jet technique wherein an aqueoussolution of a water-soluble silver salt and an aqueous solution of awater-soluble halide are simultaneously added to an aqueous solutioncontaining the protein peptizer. Precipitation is effected in an acidicmedium and should be effected in the substantial absence of anysulfur-sensitizing compounds. For this reason, I have found that mostuseful results are obtained using a photographically inert proteinpeptize-r of the type described by Mees in The Theory of thePhotographic Process, Macmillan Company, 1954, page 112. Such inertprotein peptizers are also mentioned by Lowe, Jones and Roberts inFundamental Mechanisms of Photographic Sensitivity, edited by J. W.Mitchel, Butterworth, London, 1951, page 112 (Communication 1311). Mostadvantageously, the precipitation is effected in the presence of a smallamount of a silver halide solvent, such as a water-soluble thiocyanate.The silver halide solvent or ripening agent can be added to the makingkettle before adding the soluble silver salt or it may be added with thewater-soluble halide, which is run in during the making, or preferablysome silver solvent may be present in the making kettle and some in thehalide solution.

Rapid stirring is employed during the precipitation and by using thedouble-jet technique, an exceptionally narrow grain size spread isobtained. A moderate amount of silver halide solvent further facilitatesthe formation of silver halide grains of substantially uniform grainsize. Alkali metal thiocyanates and cyanates have been found to beparticularly useful silver halide solvents. By stirring the ingredientsof the making kettle and using a silver halide solvent, it is possibleto obtain silver halide grains exhibiting particularly useful print-outcharacteristics and having -a mean grain area between 0.8, and 25,13.

The pH of the precipitation is quite important in obtaining print-outemulsions according to my process, and I have found that the pH shouldbe maintained at no more than about 5.0, and preferably less than thisfigure. Precipitation is effected in this acidic medium so that thehalide ions are always in excess during the precipitation. To achievethis, soluble halide can be added to the aqueous solution containingpeptizer before commencing the addition of the silver salt. The actualprecipitation time can vary considerably depending upon the grain sizedesired. Generally 20 to 60 minutes are suflicient to obtain an averagegrain size in the range desired. The size of the jets admitting theaqueous solutions can be varied, but they are generally similar in sizeso that the concentrations of the reactants in the making kettle can beaccurately controlled.

The amounts of soluble halide and silver salts added can be varied,depending upon the amount of silver halide to be formed. Theconcentrations of the solutions and the times taken to add them can bevaried widely. However, the time taken for the addition of the twosolutions and the concentrations of the soluble salts should besufficient to produce a coarse-grained emulsion by the end of the makingperiod.

The soluble halide employed can be, for example, all bromide or bromidetogether with a small amount of another halide, such as iodide or amixture of iodide and chloride. Alkali metal halides, such as sodium orpotassium, are particularly useful, although ammonium halides can alsobe employed. The silver salt is usually silver nitrate, athough otherwater-soluble silver salts, such as the acetate, etc., can be used.

The amount of soluble silver halide solvent employed may be varied,depending upon the particular grain size materials.

desired, etc. Thiocyanates are particularly useful silver halidesolvents and it has been found that good results can be obtained whenthe thiocyanate used ranges from 0.75 to 7.5 molar percent of the totalsilver halide formed. If thiocyanate is added With the halide solution,an amount up to 20 mole percent of the silver halide formed can be used.

The temperature of the making kettle is preferably somewhat higher thannormally used in making con ventional emulsions, since the highertemperatures facilitate grain growth. I have found that temperatures'within the range of 65 to 85 C. are particularly useful.

After precipitation has been completed, and further ripening, ifnecessary, the emulsion is cooled to about 40 to 50 C. and then mixedwith more protein peptizer, such as gelatin, and other additives, suchas coating aids. The emulsion is then ready for coating, which can beeffected without washing the emulsion, although washing with water toremove soluble salts may sometimes be used.

In the emulsions of the present invention, thiocyanates are particularlyuseful in the precipitation, since they not only function as a ripeningagent, but as halogen acceptors. If desired, supplementary halogenacceptors can be used, and where other ripening agents have beenemployed, it is particularly useful to employ a supplementary halogenacceptor. Typical halogen acceptors which can be employed in myinvention include phenylenediamines, hydroquinone, borax, alkali metalhydroxides, 3-pyrazolidones (e.g., 1-phenyl-3-pyrazolidone, etc.),thiosalicylic acid, etc.

In a particularly useful embodiment of the invention, a Water-solublelead salt can be added to the making kettle, preferably with the solublesilver salt. The lead salt has been found to provide lower backgrounddensity while exhibiting higher trace density than obtainable withemulsions containing no lead salt. Typical lead salts useful in myinvention include lead acetate, lead nitrate, etc.

The process of forming print-out images using the emulsions of myinvention comprise exposing a layer of the emulsion to a high intensityactinic light source for a very short period, followed byphotodevelopment of the latent image by uniformly exposing the layer fora second or more to diffuse light or relatively low intensity, generallyof from about 25 to 1000 ft. candles. The emulsion layer can be exposedto an oscillograph spot or to a rapidly moving galvanometer spot of thetype described in Heiland US. Patent 2,580,427, issued January 1, 1952.Upon photodevelopment with diffuse illumination, the recorded imagerapidly appears.

Recording materials made according to this invention have satisfactoryaccess time, discrimination, writing speed and stability of trace uponprolonged exposure and show at least one or more of thesecharacteristics to a substantially greater degree than heretofore knownrecording A particular advantage of my emulsions is that discriminationis much less dependent upon the intensity level of the light used forphotodevelopment, thus materially shortening the exposure required toproduce an image. Frequently, one second is all that is needed toproduce a useful image.

The following examples will serve to illustrate more fully the method ofpracticing my new invention, although i it is not to be understood thatthese examples are limittions containing 39 g. of silver nitrate in 200ml. of water and 29.1 g. of potassium bromide in 200 ml. of watersimultaneously to a solution containing 8 g. of inert "gelatin, 0.75 g.of potassium bromide, 3.0 g. of potassium thiocyanate, 5.6 ml. of 1,0N-sulfuric acid and 113 ml.

of water at 72 C. over a period of 40 minutes. The emulsion was coatedon paper and air-dried Without washing.

A sample coating of this emulsion was given an exposure to a Microflashlamp for a period of about 2 microseconds and the image photodevelopedby an exposure to diffuse daylight. It was found to photodevelop rapidlyand gave a high trace/ background density ratio, the trace being deepblue and the background light yellow. The exposed and photodevelopedmaterial was held for several hours in daylight. The trace of thematerial remained clearly visible. The sensitivity of the microflashexposure was high.

Example 2 A silver halide emulsion was prepared by adding solutionscontaining 39 g. of silver nitrate in 200 ml. of water and 28.6 g. ofpotassium bromide together with 6 g. of potassium thiocyanate in 200 ml.of Water simultaneously to a solution containing 4 g. of inert gelatin,0.465 g. of sodium chloride, 5.6 ml. of 1.0 N-sulfuric acid and 115 ml.of Water at 72 C. over a period of 40 minutes.

A sample coating of this emulsion showed photographic properties similarto those described in Example 1.

Example 3 The addition of halogen acceptors, such as thiosalicylic acidto these emulsions of Examples 1 and 2, improved their print-outcharacteristics by giving 'higher discriminations and shorter accesstimes.

Example 4 An emulsion was prepared according to Examples 1 and 2 exceptthat the silver nitrate solution contained 0.2 g. of lead nitrate. Whencoated and tested, the material containing the lead salt was found tohave a lower background density and a higher trace density than asimilar emulsion which contained no lead salt.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scop of theinvention as described herein-above and as defined in the appendedclaims.

What I claim as my invention and desire secured by Letters Patent of theUnited States is:

1. A method of preparing photographic emulsions comprisingsimultaneously adding an aqueous solution of a Water-soluble silver saltand an aqueous solution of a water-soluble halide salt, wherein at least90 mole percent of the halide salt is a bromide salt, to an aqueoussolution containing a protein peptizer thereby'precipitating silverhalide grains, maintaining the pH of said peptizer solution acidicduring said additions and maintaining an excess of halide ions oversilver ions by adding said watersoluble silver salt solution at such arate as to provide said excess of halide ions over said silver ions,said precipitation being effected in the substantial absence of sulfursensitizers and at least one of said aqueous solutions containing asmall amount of an alkali metal thiocyanate silver halide solvent toeffect ripening of said silver halide grains.

2. A method as defined in claim 1 wherein a temperature of from 65 to C.is maintained during precipitation.

3. A method as defined in claim 1 wherein the pH is maintained at notmore than 5.0 during precipitation of said silver halide.

4-. A method of preparing photographic emulsions comprisingsimultaneously adding an aqueous solutionof a water-soluble silver saltand an aqueous solution of a Water-soluble halide salt, wherein at leastmole percent of the halide salt is a bromide salt, to an aqueoussolution containing gelatin thereby precipitating silver halide grains,maintaining the pH of said gelatin solution acidic during said additionsand maintaining an excess of halide ions over silver ions by adding saidwater-soluble silver salt solution at such a rate as to provide saidexcess of halide ions over said silver ions, said precipitation beingeffected in the substantial absence of sulfur sensitizers and at leastone of said aqueous solutions containing a small amount of an alkalimetal thiocyanate silver halide solvent to efiect ripening of saidsilver halide grains.

5. A method as defined in claim 4 wherein said halide salt consists ofan alkali metal bromide.

6. A method of preparing photographic emulsions comprisingsimultaneously adding an aqueous solution of a water-soluble silver saltand an aqueous solution of a Water-solubl halide salt, wherein at least90 mole percent of the halide salt is a bromide salt, to an aqueoussolution containing gelatin there'by precipitating silver halide grains,maintaining the pH of said gelatin solution acidic during said additionsand maintaining an excess of halide ions over silver ions by adding saidWater-soluble silver salt solution at such a rate as to provide saidexcess of halide ions over said silver ions, precipitation beingeffected in the substantial absence of sulfur sensitizers, at least oneof said aqueous solutions containing a small amount of an alkali metalthiocyanate silver halide solvent to effect ripening of said silverhalide grains, and at least one of said aqueous solutions containing asmall amount of a water-soluble lead salt.

7. A method of precipitating photographic emulsions comprisingsimultaneously adding an aqueous solution of a water-soluble silver saltand an aqueous solution of a water-soluble halide salt, wherein at least90 mole percent of the halide salt is a bromide salt, to an aqueoussolution containing a protein peptizer and a water-soluble alkali metalbromide, thereby precipitating silver halide grains, maintaining the pHof said peptizer solution acidic during said additions and maintainingan excess of halide ions over silver ions by adding said Water-solublesilver salt solution at such a rate as to provide said excess of halideions over said silver ions, said precipitation being effected in thesubstantial absenc of sulfur sensitizers and at least one of saidaqueous solutions containing a small amont of an alkali metalthiocyanate silver halide solvent to efiect ripening of said silverhalide.

8. A method of preparing photographic emulsions comprisingsimultaneously adding an aqueous solution of a Water-soluble silver saltand an aqueous solution of a Water-soluble halide salt, wherein at least90 mole percent of the halide salt is a bromide salt, to an aqueoussolution containing gelatin thereby precipitating silver halide grains,maintaining the pH of said gelatin solution acidic during said additionsand maintaining an excess of halide ions over silver ions by adding saidwater-soluble silver salt solution at such a rate as to provide saidexcess of halide ions over said silver ions, said precipitation beingefiected in the substantial absence of sulfur sensitizers and at leastone of said aqueous solutions containing a small amount of an alkalimetal thiocyanate silver halide solvent to elfect ripening of saidsilver halide grains, and adding a halogen acceptor to said photographicemulsion prior to coating thereof.

References Cited by the Examiner UNITED STATES PATENTS 2,401,051 5/1946Crouse et al. 9694 2,584,029 1/1952 Land 96-29 2,618,556 11/1952Hewitson et al. 9695 2,960,405 11/1960 Dersch 96-94 3,033,682 5/1962Hunt 96-94 OTHER REFERENCES Ammann-Brass: Chimia (Switzerland), 1956,10, 173- 188 (abstracted in Chemical Abstracts, 1957, 51, 113, and inThe British Journal of Photography, Aug. 2, 1957, page 451 underAnalecta).

James et al.: Fundaments of Photographic Theory, 1948, John Wiley & SonsInc., New York, pp. 21-25.

The Journal of Chemical Education, vol. 8, No. 12, December 1931, pp.2347-2367.

Wall: Photographic Emulsions, 1929, American Publishing 00., Boston,page -59.

NORMAN G. TORCHIN, Primary Examiner.

A. D. RICCI, Assistant Examiner.

6. A METHOD OF PREPARING PHOTOGRAPHIC EMULSIONS COMPRISING SIMULTANEOUSLY ADDING AN AQUEOUS SOLUTION OF A WATER-SOLUBLE SILVER SALT AND AN AQUEOUS SOLUTION OF A WATER-SOLUBLE HALIDE SALT, WHEREIN AT LEAST 90 MOLE PERCENT OF THE HALIDE SALT IS A BROMIDE SALT, TO AN AQUEOUS SOLUTION CONTAINING GELTIN THEREBY PRECIPITATING SILVER HALIDE GRAINS, MAINTAIING THE PH OF SAID GELATIN SOLUTION ACIDIC DURING SAID ADDITIONS AND MAINTAINING AN EXCESS OF IONS OVER SILVER IONS BY ADDING SAID WATER-SOLUBLE SILVER SALT SOLUTION AT SUCH A RATE AS TO PROVIDE SAID EXCESS OF HALIDE IONS OVER SAID SILVER IONS, PRECIPITATION BEING EFFECTED IN THE SUBSTANTIAL ABSENCE OF SULFUR SENSITIZERS, AT LEAST ONE OF SAID AQUEOUS SOLUTIONS CONTAINING A SMALL AMOUNT OF AN ALKALI METAL THIOCYANATE SILVER HALIDE SOLVENT TO EFFECT RIPENING OF SAID SILVER HALIDE GRAINS, AND AT LEAST ONE OF SAID AQUEOUS SOLUTIONS CONTAINING A SMALL AMOUNT OF A WATER-SOLUBLE LEAD SALT. 